Investigation of nonlinear accelerated degradation mechanism in fuel cell stack under dynamic driving cycles from polarization processes

The nonlinear accelerated degradation severely limits the lifetime of fuel cells. This paper presents a comprehensive analysis of polarization processes during accelerated performance degradation. The double trap model is applied to reveal kinetic degradation, and an equivalent circuit model based on the distribution of relaxation times is proposed to identify and quantify the polarization loss and low-frequency inductive loop. Combined with monitoring dynamic voltage, the results show that the accelerated performance decline is dominated by the growth of platinum oxides rather than kinetic degradation and structural damage. The platinum oxides reduce the cathode mixed potential gradually, leading to the accelerated decay occurring earlier at low current density. Meanwhile, the high oxide coverage hinders oxygen diffusion and water removal, resulting in sudden and drastic concentration polarization in the high current density region. Furthermore, the formation and reduction of platinum oxide affect voltage stability and steady performance of loading and unloading processes due to their potential dependency. The present study helps in modeling and predicting the nonlinear accelerated ageing to improve the lifetime of fuel cells.